“In practical life we are compelled to follow what is most probable ; in speculative thought we are compelled to follow truth.”
– Baruch Spinoza
WARNING! The following post is highly speculative. If you consider yourself a pure scientist, do not read it.
After several days with constant showers and strong westerlies (= no ringing), I’ve got plenty of time to read random papers. It’s actually a completely different activity than reading papers “at work”; if you are not interested in what you are reading, you just leave it and go for another one. What usually happens is that you start reading about migration and end up with evolutionary ecology. Coincidence? I don’t think so.
This week, however, it was the oposite for me. I came across a couple of papers about niche filling, diversification and stability (Stigall 2014 and Price et al 2014) and they were the starting signal of what led me to read about Bar-tailed godwit migration. Since the same weather that forces me to read also forces the godwits to stop in the Peninsula, when the rain gave a break I went to Nabben to enjoy big numbers of them in beautiful summer plumage. Sometimes theory plus field observations equals conclusions and, since here in my blog I’m allowed to be especulative, let’s theorize about Nabben’s Bar-tailed godwits.
The first flock I saw was peacefully feeding in the [for once in a lifetime] quiet golf course and what stroke me in the beginning was the overall size difference between the brighter summer-plumaged and the grey winter-plumaged individuals, the latter being bigger. Differences were especially noticeable in bill lenght and depth. As usual in waders, females have a longer and deeper bill and are duller in plumage, so in the photo below there is 1 female and 4 males. The whole flock was made out of 12 males and 2 females. What’s the reason for such a biased sex-ratio? According to BWP (Cramp et al. 1983), there is not a big difference in parental care in Bar-tailed godwit, so both male and female stay with the youngsters until they fledge. Hence, there shouldn’t be a sex-related difference in departure dates. The reason might have something to do with their feeding ecology and subsequent sexual segregation. Several studies (e.g. Both et al. 2003, Catry et al. 2012) have pointed out a difference in foraging habitat due to larger food requirements in females (because of their bigger size) and they are the ones choosing the place. Seemingly, females usually forage along the rich waterline, whereas males have to get along with exposed mudflats and grassy meadows. A golf course is probably not the best place to find food so it’s not that surprising that that flock was mainly made out of males.
Let’s focus on males for a while. Their bill is more curved than that on females, and impression accentuated by its shorter lenght. Ferns & Siman 1994 studied feeding techniques and resources and related them to bill shape and found that curved bills were useful to get food from cavities and confined spaces. Makes sense! Males are feeding in poorer habitats and have a stronger need to actually look for food instead of just find it as females do in richer areas with their straighter bill. Moreover, there was a difference in bill shape even within males. I’ve tried to sort the different kinds of bill I saw in males in the crappily made collage below. In the top, bills that looked almost straight; in the middle, bills that looked straight but curved in the end; in the bottom, bills that looked wholly curved. I know, differences are subtle, but, quoting whatever famous documentarist “in nature, a milimiter makes the difference between life and death”. Don’t google it, I just made it up, but it sounds likely, doesn’t it?
Anyway, guess what, I don’t know about the bird in the top left, but the bird in the top right showed the wornest primaries among the flock, especially the outer ones in what gave the impression of a moult limit. Moreover, and always among the males, it was the one with a duller plumage despite it didn’t show any winter body feather. So it’s my strong opinion that this bird is a 1st summmer: worner primaries (maybe fresh inner due to an earlier, before migration, moult) and duller plumage than adults. Check the difference in wear level in outer primaries in the photo below, also head and body plumage. The conclusion I temptatively get from this is that the bill gets more curved with age. It also makes sort of sense: females will be always dominant, pushing males towards a place where they need a curved bill. Lammarck would say young birds change the shape of the bill to make it fit with the habitat they are foraging in. Maybe… but a modern approach would probably say that only juveniles with curved bills reach the adult age. Good luck to our misfit friend!
Let’s now focused on putative adult males and their shinny bellies. Piersma & Jukema 1993 suggested that the brightness of the orange in summer-plumaged Bar-tailed godwit was an honest signal in relation to migratory skills. Birds start with their pre-breeding (pre-alternate for the yanks) moult in winter quarters, they suspend it before departing and then congregate in places such as the Wadden Sea (north central Europe) to finish it. Despite moult only takes an extra 7% of energy, only the individuals in good physical condition (this is, more skillful in terms of feeding or flying performances) can afford it. The bird in the right of the photo below is obviously brighter than the other two, despite all three of them presumably belong to the same age and sex classes. I wonder if it has something to do with the mentioned study…
And now we are talking about migration, it’s worth saying that Bar-tailed godwit still holds the longest non-stop flight record: 11.500 km. from Alaska to New Zealand! Hedenström 2010 tries to answer at least some of the thousands of questions that emerge from this record, but ends up saying that not even the extremely especialized physiology of Bar-tailed godwits can explain such a success. Maybe some help from the weather? Let’s leave this questions for the next wave of rain…